System for indicating aerosols in the atmosphere

ABSTRACT

A fire detector having an ionization sensing means which changes the level of an output signal in accordance with sensing the presence of smoke in the atmosphere has been provided. A detecting means selectively responsive to the output signal generates an alarm control signal and the improvement includes a filter means for rendering a detecting means operable if uninhibited to generate the alarm signal irrespective of an output signal from the sensing means. Inhibiting means normally operable to inhibit the filter is rendered ineffective in response to the output signal of the sensing means and the inhibiting means therefore checks the operability of the detecting means.

United States Patent Tipton et al.

[ Apr. 17, 1973 [5 SYSTEM FOR INDICATING AEROSOLS 2,646,556 7/ I953Allen ..340/237 SX IN THE ATMOSPHERE [75] Inventors: William C. Tipton,Newark; Michael gsx zf l glg g' fjfi j Mountamslde both ofAttorney-Harold S. Wynn and John P. DeLuca [73] Assignee: General SignalCorporation, ABSTRACT RochesterNY' A fire detector having an ionizationsensing means [22] Filed; se nzg, 1970 which changes the level of anoutput signal in ac- 2 cordance with sensing the presence of smoke inthe at- I 1] Appl' 76149 mosphere has been provided. A detecting meansselectively responsive to the output signal generates an 52 US. Cl...340/237 s, 250/83.6 FT alarm control Signal and the improvementincludes a 511 lm. c1. ..G08b 17/10, H01 j 39/28 filter means forrendering a detecting means Perable 58 Field of Search ..340/237 sifuninhibited generate the alarm Signal irrespective 250/435 D 44 of anoutput signal from the sensing means. Inhibiting means normally operableto inhibit the filter is 56] References Cited rendered ineffective inresponse to the output signal of the sensing means and the inhibitingmeans therefore UNITED STATES PATENTS checks the operability of thedetecting means.

3,462,752 8/1969 Stroh ..340/237 S 15 Claims, 4 Drawing Figures OTHERDETECTOR AMPL'F'ER INHIBIT FILTER 12 62 5 025 27 L 37 35 4| 44 59 u L G229 j 2% I 3 G y g 5 5 [I49 I |:)s u )aAl 33 39 .12 l 2 1 11 $53 I 2|l2'3g" 32 N /43 Z @Q 26 G5\ l .J 38 46 4s 47 IP 28 1 46 %45 n- 5's I7DETECTOR II L SYSTEM FOR INDICATING AEROSOLS IN THE ATMOSPHEREBACKGROUND OF INVENTION This invention relates to a tire detector systemand in particular to the system for detecting the presence of combustionproducts, aerosols in the atmosphere.

Modern fire detection equipment generally utilizes an ionization chamberwhich uses a radio-active source to ionize the atmosphere within thechamber and a 1 and reduce the current through the chamber.Otherfactors, however, may provide a variation in voltage which wouldgive false indication of a fire condition.' A draft, for example, causedby a slamming door might disturb the atmosphere within the chamber suchthat an alarm is sounded. Variations in atmospheric pressure also effectthe characteristic of the chamber. As is well known in the art, twochambers can be used; one open to the atmosphere for receiving ordetecting the presence of aerosols and another substantially closed toatmospheric aerosols. The closed chamber is a reference for compensatingthe system under changes in the atmospheric pressure. The system stillmay, however, be susceptible to false triggering and noise transientswhich may be received by the equipment.

Generally it is desirable to connect a plurality of fire detectors in aspecific area and link them together to a receiver which would activatethe alarm signal. However, noise, loading and supervisory problems havemade installing such fire detectors on a system-wide basis with acentral indication very difficult and costly.

In order to provide for a truly safety oriented system, supervision ofthe circuits joining multiple sensing units and individual troubledetection in each of the units is necessary. Trouble may be corrected assoon as it occurs. Discovery should not be delayed by the frequency ofperiodic checking, but rather continuous monitoring of the firedetection apparatus should be conducted.

It is therefore an object of the present invention to provide anarrangement which substantially obviates one or more of the difficultiesof the described prior arrangements.

It is another object of the present invention to provide a. simplifiedfire detection system.

It is yet another object of the present invention to provide a safe firedetection system.

It is still another object of the present invention to provide aneconomically manufactured installed and maintained fire detectionsystem.

SUMMARY OF INVENTION There has been provided a fire detection systemincluding ionization sensing means for changing the level of an outputsignal in accordance with sensing the presence of smoke in theatmosphere. Detecting means selectively responsive to the output signalgenerates an alarm control signal and the improvement includes filtermeans for rendering the detecting means operable if uninhibited togenerate the alarm signal irrespective of an output signal for thesensing means. Inhibiting means including a voltage level detector isalso provided which is normally operable to inhibit the filtering meansand it is rendered ineffective when a voltage level governed by theoutput signal is above or below a predetermined normal voltage range.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, whileits scope will be pointed out in the appended claims.

FIG. 1 is a schematic diagram of the fire detector including the sensingmeans, detector circuitry and the improved apparatus of the presentinvention.

FIG. 2 is a diagram of the control unit which may be remotely locatedfor central monitoring of a number of detectors coupled in parallel.

FIG. 3 is a diagram of the supervisory circuit coupled in parallel tothe last unit in the multiple detector system.

FIG. 4 is a drawing showing the structure of the sensing unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT The combustion products smokedetection system consists essentially of three units; a detector 11,control unit 15, and an end-of-line signal sender 16.

A detector unit 11 produces an output when combustion products aresensed by an ionization chamber 10.

A control unit 15 supplies regulated, short circuit protected, 16V D.C.,to operate from one to 10 detectors connected in parallel. In addition,it indicates a trouble condition if the line continuity is broken, or ifthe 16V D.C. is lost for any reason. FIRE indication condition isprovided when a detector 11 output, signals the presence of combustionproducts.

An end-of-line sender unit 1t; furnishes current pulses to a line Lwhich are received by a trouble detection portion of the control unit 15thus, monitoring continuity.

DETECTOR The detector 11 circuit operates on a relatively low inputvoltage in the order of 16V D.C., applied to terminals l7 and 18.Numerals 20 and 21 indicate ionization chambers of sensing means 10activated by a radio-active material 19 which emits alpha particles.

Chamber 21 is open to the atmosphere and exposed to combustionparticles. Chamber 20 is the reference chamber closed to aerosols whichis used to nullify the effects of atmospheric changes. The voltageacross the ionization chambers 20 and 21 connected in series is filteredby capacitor 22.

Particulate products of combustion entering the open sensing chamber 21hinder the normal ion current flow through the chamber caused by alphaparticles from the radio-active material 19, resulting in a rise ofvoltage across the chamber 21. The input stage of the detector 11circuitry is a dual-gate MOSFET (Metal Oxide Semiconductor Field EffectTransistor) 23, connected as a source follower. The drain D electrode ofF ET 23 is connected to the positive potential, the gate electrode G1,is connected to the junction of chambers and 21, the source electrode Sis connected to resistor 24, and the remaining electrode G2, isconnected to the source. In this connection the source S voltage followsthe voltage at gate G1, which is the chamber 21 voltage. It should alsobe noted that a resistor 24 is selected to provide stable temperaturecharacteristic to the response of PET 23 and is connected betweenelectrode S and negative terminal 17. Capacitor 25 prevents damage toFET 23 should gate G1 suddenly go positive.

The next stage of circuitry functions as a voltage level detector.Transistor 26 is a Programmable Unijunction Transistor (PUT) and withresistor 27 and capacitor 28 forms an oscillator or inhibit means.Resistor 29, rectifier 30, and resistor 24 set a reference voltage levelfor PUT 26. Capacitor 28 charges through resistor 27 until its voltageexceeds the reference level which is the voltage at the gate electrodeG3 of PUT 26 connected to resistors 29, 31 and rectifier by about 0.3V.At this point, conduction begins between the anode A1 and the cathodeCl. Capacitor 28 then discharges through the base emitter junction oftransistor 32 until its voltage drops to a cut off level at whichconduction cannot be maintained. Cut off then restores the anode Al tocathode C1 blocking of PUT 26 and capacitor 28 may recharge throughresistor 27. This charging and discharging of capacitor 28 continues,forming a free running oscillator. A voltage divider formed by resistors37, 38 and potentiometer 39 is a sensitivity adjustment for thedetector. The voltage at the slider of potentiometer 39 sets the maximumvoltage that capacitor 28 can charge to. Rectifier 33 permits dischargeof capacitor 28 if it exceeds the voltage set at potentiometer 39. ifthe voltage that capacitor 28 charges to is not greater than the gatevoltage by 0.3V, conduction of PUT 26 does not take place, andoscillations cease. This can occur if the gate G3 voltage of PUT 26 israised high enough as is the case when the sensing chamber 21 voltagerises from the entrance of particulate matter created by combustion.This, therefore, inhibits the filter means 12 described further in thediscussion. A fire condition, therefore, causes the oscillations of PUT26, resistor 27 and capacitor 28 to cease.

Each time capacitor 28 discharges through PUT 26, it biases transistor32 into conduction. Filter 12 comprises capacitor 40, resistor 41 andtransistor 32. When transistor 32 conducts, capacitor 40 which ischarged through resistor 41 discharges through rectifier 42. Sincetransistor 32 conducts periodically, capacitor 40 is dischargedperiodically. During the non-conduction periods, capacitor 40 rechargesthrough resistor 41. The discharge of capacitor 40 depends on therepetition rate of the pulses gating transistor 32 from capacitor 28,and the charge of capacitor 40 depends on the time constant ofresistor-capacitor 41-40 combination. It should be noted that resistoralso provides bias voltage for transistor 32. The voltage that appearson capacitor depends on the relation of this repetition rate to theresistor'capacitor 41-40 time constant. The average value of voltage ontransistor 32 is in the order of 2V. The resistor-capacitor 41-40combination in cooperation with transistor 32 therefore filters outsignals ofless than a predetermined rate.

Transistor 43 is a Programmable Unijunction Transistor (PUT) withresistors 44 and 45 setting the voltage reference level on gate G4. Ifthe pulses which make transistor 32 conduct, stop, capacitor 40 willcharge to a higher voltage than the average. When this voltage exceedsthe reference set by resistors 44 and 45 by approximately 0.3V,conduction takes place from anode A2 to cathode C2 and capacitor 40discharges through PUT 43 and resistor 46. The voltage across resistor46 is coupled to the gate GS of Silicon Controlled Rectifier SCR 47through resistor 48 and triggers conduction from its anode A3 to cathodeC3. This then effectively grounds resistor 49 and lights a fireindication lamp 50, and also any external light (not shown) that may beconnected across terminals 51 and 52. Resistors 49 and 53 are droppingresistors for low voltage lamps such as lamp 50. Resistor 54 andcapacitor 56 form a filter network to prevent firing of (SCR) 47 fromspurious transients. Rectifier 57 allows the fire indication lamp tolight immediately if the line should be connected in reverse polarity.Capacitor 58 is a filter capacitor to filter R.F. transients from theunit. Diode 62 coupled between capacitor 58 and resistor 44 is providedfor polarity protection for the semi-conductors if the apparatus of FIG.1 is inadvertently connected in reverse. Resistors 59 and 60 assureclosure of the Fire Sense Relay 61 in the control unit 15 should thefire indication lamp 50 be open circuited.

The control unit 15 contains the voltage regulated supply, fire sensingcircuitry, trouble circuitry, power failure sensing and emergency powertransfer circuitry.

POWER INPUT Normal power is supplied to the control unit through asuitable fuse 156 and transformer 67 which is full wave rectified bybridge 68 and filtered by capacitor 69. Emergency battery power isapplied to terminal 70 and externally switched, full-wave rectified DC.power may be applied to terminal 71. Resistor 77 limits the surgecurrent due to the charging of the filter capacitor 69. When AC power isapplied, relay 73 is energized thus opening normally closed contacts 74and closing normally open contacts 75. This opens the battery input andconnects the AC power to the control unit. In addition, if A.C. power islost for any reason, relay 73 deenergizes and closes contacts 152 forproducing an A.C. power loss signal. Diode 76 blocks battery voltagefrom re-energizing relay 76 through resistor 77 if AC power is lost.

VOLTAGE REGULATOR Zener diode 78 and resistor 79 form a voltage divider.The zener voltage of 78 is about 10V. Resistors 80 and 81 in parallelwith zener diode 78 furnish a reference voltage for the base oftransistor 82. Since the base voltage is fixed, the drop across resistor83 is also fixed producing constant emitter current. Neglecting basecurrent the emitter and collector currents are equal and fixed andtransistor 82 forms a constant current generator. The collector currentof transistor 82 supplies the base of transistor 84 and the collector oftransistor 85. Transistors 84 and 86 are Darlington connected currentamplifiers. Zener diode 87 and resistor 88 are in series across theoutput. The junction of zener diode 87 and resistor 88 is atapproximately 10V, with respect to ground, and is a reference for theemitter of transistor 85. Resistors 89, 90 and 91 form a voltage divideracross the output of transistor 86. Resistor 90, which is adjustable, isused to vary the output. The output is varied in the same manner thatthe output is regulated. If the slider on resistor 90 is made morepositive, or the output voltage increases, the base current oftransistor 85 is increased, which causes 85 to conduct more. Since thecollector current of transistor 82 is constant, the additional currentmust come from a decrease in base current of transistor 84 and in turn adecrease in the collector current of transistor 86. This decrease incurrent is accomplished by an increase in drop across 86. The outputvoltage will therefore decrease, since transistor 86 is in series withthe supply and load. For decrease in the slider voltage of resistor 90or a decrease in output voltage, the opposite sequence occurs with aresulting increase in the output voltage. Capacitor 92 is used as afilter capacitor.

OVER-CURRENT PROTECTION Over-Current Protection is provided to preventdamage due to an accidental short circuit of the output terminals.Output current passes through resistor 93 which is connected to theemitter of transistor 94 and base of transistor 94 through current limitresistor 95. An increase in output current of transistor 86 increasesthe drop across resistor 93 and increases the current through transistor94 until saturation occurs. The increased current draws base currentfrom transistor 84 thus decreasing the drive of transistor 86 and limitsits output current.

FIRE SENSING A parallel combination of relay 61, resistors 96 and 97 isplaced in series with the detectors 11. When a detector 11 is activateddue to the presence of smoke, there is an increase in current due to theadded load of the tire indicating lights 50 and the resistors 59 and 60in the detector 11. This increase of current actuates the sensing relay61. The current drawn per detector ranges from a quiescentcurrent ofabout 2.5 ma to a fire condition current of 30 ma minimum. The relay 61is calibrated to close with an increase of ma by closing switch 100(which connects the resistor 98 and 99 across the supply) and increasingthe resistance with resistor 96 until relay 61 pulls in. This step isperformed with all the detectors 11 connected. When switch 101controlled by relay 61 closes, voltage is impressed across resistors 102and 103 applying voltage to the gate G6 of silicon control rectifier(SCR) 104 causing anode-cathode conduction. This applies voltage acrossrelay 105 energizing it. One set of contacts 109 of relay 105 isavailable for fire indication, and the other set 110 is used to shortout the fire sensing relay, 61 to protect it from over current. Diode106 prevents inductive voltage spikes when relay 105 deenergizes.Resistor 107 and capacitor 108 limit the rate of rise of voltage appliedto SCR 104 to prevent accidental triggering. Diode 111 is a Stabistor"diode used to bypass the surge currents of the detectors from relay 61.For a 0.5V pickup voltage of relay 61, diode 111 conducts less than Ima.

TROUBLE CIRCUIT The trouble circuit detects the pulses placed on theline L connecting all detectors 11 by the End of Line Sender 16 unitlocated at the last detector. The primary of transformer 112 is placedin series with the control unit 15 output terminal 113. The pulses whichappear across the secondary of transformer 112 are rectified by diode115, filtered by capacitor 116, and impressed across resistors 117 and118. A portion of this voltage is applied to the base of transistor 119.Transistor l 19 which is non-conducting is turned on by the incomingpulses. When 119 is on, capacitor 120 charges through resistor 121 andwhen transistor 119 is off, capacitor 120 discharges through resistor122. The discharge time constant is chosen to be much longer than therepetition rate of the input pulses from the sender 16. The averagevalue of the voltage at the junction of resistors 121, 122 and capacitor120 which is tied to the gate G7 of field effect transistor FET 124 isabout 8-9V. This reduces the gate voltage from the no signal condition.Since the source terminal S2 follows the gate G7 except for a smallvoltage difference, the source voltage is reduced also. Connected to thesource S2 is a series combination of resistors 125, 126 and a 10V zenerdiode 127. The source voltage under these conditions is not sufficientto overcome the zener voltage and cause conduction through this path.The junction between diode 127 and resistor 126 is at a low voltage, sothat transistor 128 is cut off with a resulting high voltage on thecollector. Transistors 129 and 130 are in series with the Trouble Relay131. In a normal ready condition, both transistors 129 and 130 will beconducting and 131 will be energized. Rectifier 139 coupled across thecoil of relay 131 acts as an electrical surge suppressing device.Transistor 129 will be conducting when transistor 128 is cut off sinceresistor 132 is supplying base current. Should the pulses fail to bereceived for any reason, transistor 129 would cut off since transistor128 would divert its base current. This would cause the Trouble Relay131 to deenergize, indicating trouble. Transistor 130 is maintainedconducting by the current through the fire lamp 133, wire 133a andresistors 134, 135 and 136. A portion of this current supplies the baseof transistor 130 to maintain it conducting. This supervisory currentthrough the lamp 133 is not sufficient to light it. If the lamp 133 isremoved or opens, transistor 130 will cut off and trouble relay 131 willbecome deenergized. In summary,

Trouble" will be indicated when relay 131 deenergizes opening normallyclosed contacts 151 for providing a trouble signal, this occurring fromloss of pulses received from the line L, the loss of the lamp 133, orthe loss of the regulated positive voltage for the system.

The diodes 137 and 138 are connected to the anode of SCR 104 which iscaused to conduct when a fire signal occurs. This places the diodes 137and 138 at near ground level as the anode of SCR 104 would then be at alow voltage. The lamp 133 with its protective fuse circuit 140 will thenlight as almost full supply voltage is across it and resistor 134, itsdropping resistor. Also, Trouble Relay 131 will be locked out givingpriority to the fire signal.

END-OF-LINE SENDER The End-of-Line Sender 16 generates pulses and placesthem on the lines L connecting the detector 11 to the control unit 15.The unit is connected to the line at the most remote detector. Theoperation is as follows: diode 141 provides protection againstconnecting the leads in reverse polarity. Transistor 142 is a U-nijunction Transistor which, with resistor 143 and capacitor 144 formsan oscillator. The pulses are developed across resistor 145 and thedivider formed by resistors 146 and 147. The pulse voltage acrossresistor 147 is applied to the base of transistor 148. The emittercurrent of 148 supplies enough drive to the base of transistor 149 tosaturate it. Thus, almost the full l6V supply is placed across resistor150. The supply will be loaded by about 100 ma for the duration of thepulse. The pulse duration is set for about 500 microseconds and itoccurs periodically every 500 milliseconds.

P16. 4 shows a preferred structure of a typical detector 11 havingsensing means which is used to sense the presence of aerosols in theatmosphere and produce an output signal indicative of a fire condition.The sensing means includes a base number 160 which has an adjustablereference electrode 161 appropriately coupled or mounted thereto.Cylindrical housing 162 is fitted over the reference electrode 161. Acommon electrode 163, which is a bobbin shaped body of conductor havingupper and lower surfaces A and B respectively, is fit into the housing162. On each of the surfaces of the cylindrical electrode 163 isdeposited a small amount of radio-active material, which in thepreferred embodiment, is Americium 241 and is designated by thereference 164.

The reference chamber is that volume disposed between the referenceelectrode 161 and the lower surface B of the common electrode 163. Theadjustment of the distance between reference electrode 161 and surface Bsets up a reference potential. This is substantially isolated from theatmosphere such that no aerosols enter the chamber 20 and interfere withthe current produced by the radio-active source 19.

The sensing chamber 21 is formed between the upper surface A of thecommon electrode 163 and sensing electrode 168 mounted above the commonelectrode 163. This chamber is substantially opened to the atmosphereand may receive aerosol particles present in the atmospheric medium.

The upper surface A of the common electrode 163 is recessed in thecylindrical housing 162 in order to deflect radiation emitted by thesource 19 towards the sensing electrode 168. The cylindrical housing 162is composed of teflon material and serves as an absorber of alphaparticles which may be emitted at angles away from the sensing electrode168. This cylindrical housing 162, therefore, serves to columnate ordirect the radio-active emissions toward the sensing electrode 168.Another reason for the use of teflon in the construction of thecylindrical 162 is that the teflon has a very high insulative qualityand facilitates the use of PET 23 to detect extremely low level changesin ionization potential due to presence of smoke aerosols.

A printed circuit board 165 is mounted about the housing 162 to the base160 and includes the components of the electronic circuitry shown inFIG. 1. The

field effect transistor 23 is mounted to the board 165 and coupled tothe common electrode 163 through a hole in the cylinder 162.

To the base is mounted a cover 166 for protecting the circuitry andinternal components from dust and the like. Warning lamp 50 is coupledto the output of the circuitry on the printed circuit board 165. Thewarning lamp 50 is mounted in the cover 166 and gives visual indicationof a fire condition. This is convenient for checking where a fire ortrouble condition exists in a specific chain of detectors. Over thecover 166 is mounted the sensing electrode 168 and between the electrode168 and wind shield 170 is a screen 169 used to protect the sensingelectrode and to permit the free flow of atmosphere into the sensingchamber 21.

It can be seen from the drawing that the construction of the sensingunit 10 is simplified and that the reference electrode 161, sensingelectrode 168 and common electrode 163 may be coupled to the input ofthe detector circuit mounted on the printed circuit board very readily.The chambers 21 and 20 are also designed for ease of manufacture becausethey are one next to the other rather than one inside of the other as inother detectors in the art. It is apparent from the construction of thechambers in the present invention that assembly time of the unit isgreatly reduced and the efficiency of the device is unaffected.

The system shown in the present disclosure therefore provides foraccurate sensing of a fire condition as well as warning of a malfunctionin any of the units or a break in the transmission line between thecentral indication unit and any one of the sensing units remotelylocated. The system is provided with a safety feature for eliminatingthe condition whereby a false alarm is produced, and the construction ofthe sensing means has beengreatly simplified in order to reduce the costof manufacture and reduce maintenance cost by making the unit reasonablyinexpensive and readily replaceable.

While there has been shown what is considered to be the preferredembodiment of the present invention, it will be obvious to those skilledin the art that certain modifications and changes may be made withoutdeparting from the true spirit and scope of the invention, and it isintended in the appended claims to cover all such modifications andchanges within the scope of the appended claims.

What is claimed is:

1. A fire detector including; ionization sensing means for increasingthe level of an output signal relative to a normal threshold value inaccordance with sensing the presence of smoke in the atmosphere, anddetecting means selectively responsive to the output signal forgenerating an alarm control signal wherein the improvement comprises:

filter means effective if uninhibited for generating the alarm signal;and inhibiting means including a voltage level detector governed by theoutput signal, operable to inhibit the filter means only if the outputsignal is within a normal voltage range,

whereby an alarm signal is generated'when the output signal is above thevoltage range as when smoke is present in the chamber or when the outputsignal is below the voltage range as when there is a malfunction in theionization sensing means.

2. The fire detector of claim 1 wherein said detecting means comprises:a field effect transistor amplifier having its input governed by theoutput signal and switching means including a plurality of electronicgates energized in accordance with an output of the amplifier forgenerating said alarm output signal.

3. The fire detector of claim 2 wherein said inhibiting means includes:charging means coupled to a first of said gates, said first gate beingresponsive to the level of the output of the amplifier and said chargingmeans to periodically trigger said gate upon each accumulation of energybeyond the level of said output of the amplifi- 4. The fire detector ofclaim 3 wherein said charging means includes a resistor and capacitorcombination coupled to said first gate.

5. The fire detector of claim 4 wherein said filter means comprises:timing means responsive to the periodic triggering of said first gateproducing a pulse for each occurrence.

6. The fire detector of claim 5 wherein said timing means includes acharging circuit coupled between a second and third of said gates, saidcharging circuit being discharged through said second gate when saidsecond gate is in a conductance state, said second gate conductancestate being governed by said triggering of said first gate; and thethird gate being turned on to produce a signal when said charging rateis less than said triggering rate of said first gate.

7. The fire detector of claim 6 wherein said detecting means includes: afourth of said gates, operative to produce said output alarm signal inresponse to the conductance state of said third gate.

8. A system as defined in claim 1 further comprising: centrally locatedcontrol means including line wires coupled to each of a plurality ofsaid fire detectors for indicating said alarm condition.

9. A system as defined in claim 8 wherein said indicating meanscomprises: current sensitive means responsive to a change in currentoccasioned by the occurrence of said alarm signal for providing saidindication including a relay energized in accordance with said currentincrease.

10. A system as defined in claim 8 further comprising: an end of linecode sender connected across the line wires for terminating a linecircuit connecting the detectors to the centrally located control meansincluding:

an oscillator powered by energy applied to the line wires for generatingpulses for transmission over the line wires to the centrally locatedcontrol means, and

centrally located receiver means responsive to said pulses forindicating a trouble condition upon the cessation of pulses.

11. A system as defined in claim 10 wherein said receiver meanscomprises:

switching means having a conductance state in accordance with saidpulsed energy signals,

timing means including a resistor and capacitor charging circuitdischarged each time said switching means is in its conductance state,said charging circuit having a discharge time greater than the normalfrequency of said pulsed energy;

and means responsive to a charged condition of said charging means forindicating a trouble condition upon the cessation of pulses.

12. The fire detector of claim 1 wherein said ionization sensing meanscomprises: two ionization chambers coupled serially across a source oflow voltage including a reference chamber and a measuring chamber, saidreference chamber being substantially isolated from smoke particlesestablishing a reference potential for said detector and said measuringchamber serially coupled adjacent to said reference chamber forproducing a change in the level of said reference potential therebyproviding said output signal.

13. The fire detector of claim 1 comprising:

a base member;

a cylindrical tube of insulative material mounted thereto;

a common electrode plate secured coaxially within said tube, saidelectrode coupled to said detector means;

a reference electrode substantially parallel with said common electrodemovably mounted to said base for establishing the reference potential inaccordance with the spacing of said common electrode and said referenceelectrode;

a measuring electrode mounted to said base at the opposite end of saidtube in spaced relation with the opposite side of said common electrode,said measuring electrode having openings therein for admitting smokeparticles.

14. The fire detector of claim 13 wherein said ionization chamberincludes a source of alpha particle emitting material disposed on eachside of said common electrode for ionizing the atmosphere with saidchambers and producing an ionization current in accordance with apotential imposed across the reference and measuring electrodes.

15. The fire detector of claim 14 wherein said measuring electrode andthe associated side of said common electrode are in a spaced relationsuch that the walls of the tube tend to columnate the alpha particlestowards said measuring electrode for uniform ionization of theatmosphere.

1. A fire detector including; ionization sensing means for increasing the level of an output signal relative to a normal threshold value in accordance with sensing the presence of smoke in the atmosphere, and detecting means selectively responsive to the output signal for generating an alarm control signal wherein the improvement comprises: filter means effective if uninhibited for generating the alarm signal; and inhibiting means including a voltage level detector governed by the output signal, operable to inhibit the filter means only if the output signal is within a normal voltage range, whereby an alarm signal is generated when the output signal is above the voltage range as when smoke is present in the chamber or when the output signal is below the voltage range as when there is a malfunction in the ionization sensing means.
 2. The fire detector of claim 1 wherein said detecting means comprises: a field effect transistor amplifier having its input governed by the output signal and switching means including a plurality of electronic gates energized in accordance with an output of the amplifier For generating said alarm output signal.
 3. The fire detector of claim 2 wherein said inhibiting means includes: charging means coupled to a first of said gates, said first gate being responsive to the level of the output of the amplifier and said charging means to periodically trigger said gate upon each accumulation of energy beyond the level of said output of the amplifier.
 4. The fire detector of claim 3 wherein said charging means includes a resistor and capacitor combination coupled to said first gate.
 5. The fire detector of claim 4 wherein said filter means comprises: timing means responsive to the periodic triggering of said first gate producing a pulse for each occurrence.
 6. The fire detector of claim 5 wherein said timing means includes a charging circuit coupled between a second and third of said gates, said charging circuit being discharged through said second gate when said second gate is in a conductance state, said second gate conductance state being governed by said triggering of said first gate; and the third gate being turned on to produce a signal when said charging rate is less than said triggering rate of said first gate.
 7. The fire detector of claim 6 wherein said detecting means includes: a fourth of said gates, operative to produce said output alarm signal in response to the conductance state of said third gate.
 8. A system as defined in claim 1 further comprising: centrally located control means including line wires coupled to each of a plurality of said fire detectors for indicating said alarm condition.
 9. A system as defined in claim 8 wherein said indicating means comprises: current sensitive means responsive to a change in current occasioned by the occurrence of said alarm signal for providing said indication including a relay energized in accordance with said current increase.
 10. A system as defined in claim 8 further comprising: an end of line code sender connected across the line wires for terminating a line circuit connecting the detectors to the centrally located control means including: an oscillator powered by energy applied to the line wires for generating pulses for transmission over the line wires to the centrally located control means, and centrally located receiver means responsive to said pulses for indicating a trouble condition upon the cessation of pulses.
 11. A system as defined in claim 10 wherein said receiver means comprises: switching means having a conductance state in accordance with said pulsed energy signals, timing means including a resistor and capacitor charging circuit discharged each time said switching means is in its conductance state, said charging circuit having a discharge time greater than the normal frequency of said pulsed energy; and means responsive to a charged condition of said charging means for indicating a trouble condition upon the cessation of pulses.
 12. The fire detector of claim 1 wherein said ionization sensing means comprises: two ionization chambers coupled serially across a source of low voltage including a reference chamber and a measuring chamber, said reference chamber being substantially isolated from smoke particles establishing a reference potential for said detector and said measuring chamber serially coupled adjacent to said reference chamber for producing a change in the level of said reference potential thereby providing said output signal.
 13. The fire detector of claim 1 comprising: a base member; a cylindrical tube of insulative material mounted thereto; a common electrode plate secured coaxially within said tube, said electrode coupled to said detector means; a reference electrode substantially parallel with said common electrode movably mounted to said base for establishing the reference potential in accordance with the spacing of said common electrode and said reference electrode; a measuring electrode mounted to said base at the opposite end of said tube in spaced relation with the oppositE side of said common electrode, said measuring electrode having openings therein for admitting smoke particles.
 14. The fire detector of claim 13 wherein said ionization chamber includes a source of alpha particle emitting material disposed on each side of said common electrode for ionizing the atmosphere with said chambers and producing an ionization current in accordance with a potential imposed across the reference and measuring electrodes.
 15. The fire detector of claim 14 wherein said measuring electrode and the associated side of said common electrode are in a spaced relation such that the walls of the tube tend to columnate the alpha particles towards said measuring electrode for uniform ionization of the atmosphere. 